1. Field of the Invention
The present invention relates to an operational amplifier, and more particularly, to an operational amplifier utilizing depletion-type metal-oxide-semiconductor field effect transistors (MOSFETs) as an input pair, and capable of only utilizing an input pair to achieve a rail to rail output voltage and reduce circuit area.
2. Description of the Prior Art
An operational amplifier is a widely used element for realizing a variety of circuit functions. Taking driving circuits of a liquid crystal display (LCD) for example, the operational amplifier can be used as an output buffer, which charges or discharges loading ends, i.e. liquid crystals, according to analog signals outputted by a front stage digital to analog converter (DAC), for driving corresponding pixel units on the LCD.
In a conventional driver chip, the operational amplifier generally has a two-stage structure, which includes a first stage amplification circuit (input stage) and a second stage output circuit (output stage). The first stage amplification circuit is utilized for increasing current or voltage gain of the operational amplifier, while the second stage output circuit is utilized for driving capacitive or resistive loads connected to the operational amplifier. In addition, since the operational amplifier may suffer loop instability problems, Miller compensation capacitors are commonly implemented to perform frequency compensation for improving loop stability.
Besides, since a P-channel input pair and an N-channel input pair can only respectively adapt to different specific operating voltage ranges, and thus the conventional operational amplifier usually utilizes both the P-channel input pair and the N-channel input pair as an input stage to achieve a rail to rail output voltage. For example, please refer to FIG. 1A and FIG. 1B. FIG. 1A is a schematic diagram of a part of a conventional operational amplifier 10, and FIG. 1B is a schematic diagram of an operating voltage of the operational amplifier 10 shown in FIG. 1A. As shown in FIG. 1A, the operational amplifier 10 includes input pairs 100, 102, active loads 104, 106 and bias sources 108, 110, wherein the P-channel input pair 100 includes P-channel enhancement-type MOSFETs MP1, MP2 and the N-channel input pair 102 includes N-channel enhancement-type MOSFETs MN1, MN2, and the input pairs 100, 102 receives input voltages In+, In− for the operational amplifier 10 to generate an output voltage accordingly.
In such a situation, as shown in FIG. 1A and FIG. 1B, if the transistor MP2 is intended to be turned on, a system voltage VDD minus a turned on voltage VDsat of the bias source 108 and an inverse number of a threshold voltage of the transistor MP2 need to be greater than the input voltage In+ (i.e. the threshold voltage of the P-channel enhancement-type transistor is a gate-to-source voltage and a negative value, and thus a source-to-gate voltage VSG need to be greater than the inverse number of the threshold voltage for the P-channel enhancement-type transistor to be turned on), and the transistor MP1 requires the same operating condition. Therefore, the P-channel input pair 100 can only be turned on in a range CMP; similarly, if transistor MN2 is intended to be turned on, the input voltage In+ minus a threshold voltage of the transistor MN2 and a turned on voltage VDsat of the bias source 110 should be greater than a ground voltage level GND, i.e. 0V (i.e. the threshold voltage of the N-channel enhancement-type transistor is a gate-to-source voltage and positive value, and thus a gate-to-source voltage VGS need to be greater than the threshold voltage for the N-channel enhancement-type transistor to be turned on), and the transistor MN1 requires the same operating condition. Therefore, the N-channel input pair 102 only can only be turned on in a range CMN. As a result, since the operational amplifier 10 includes both the P-channel input pair 100 and the N-channel input pair 102 for the ranges CMP, CMN, the operational amplifier 10 can operate in a full range VCM and achieve a rail to rail output voltage.
However, the conventional operational amplifier 10 needs to include both the P-channel input pair 100 and the N-channel input pair 102, and requires larger circuit area. Therefore, there is a need for improvement of the prior art.